St. Croix Medical has developed a fully implantable hearing aid that has completed Phase 1 U.S. clinical trials. The device consists of an indwelling, self-powered sound processor and separate piezoelectric transducers to sense incus vibrations and drive the stapes. It is intended for people with moderate to severe sensorineural hearing loss of cochlear origin. The device's advantages are that it preserves the resonance and filter properties of the external ear, contains no external components, and should last >4 years before the battery requires replacement. The current-generation implant has been installed in 13 patients to date. Early clinical results indicate that despite intraoperative measures that consistently predict adequate device performance, functional gain at activation is nonetheless highly variable (0-22 dB). The situation mirrors one that commonly occurs with partial and total ossicular chain prostheses used in other procedures. Factors underlying variability include inefficient transducer coupling to the ossicular chain and an inability of the unions to accommodate postoperative shifts in position. The goal of this work is to develop new prosthesis coupling methods whose performance meets or exceeds that of the natural, unimplanted ossicular chain. A novel, low-cost, piezoelectric-based force measurement tool that operates in the micronewton range will be developed. It will be used to obtain first-ever measures of compliance/damping characteristics of ossicular chain ligaments and joints in human temporal bones. Based on these measures, biocompatible adhesives whose physical properties match those of middle ear structures will be selected and characterized in bench and temporal bone studies. Initial candidates include silicone medical adhesive, fibrin glue and BioGlue. A novel "soft contact" transducer coupling that accommodates small shifts in contact geometry will also be developed as an alternative to bonding a transducer to the stapes. Phase I of the project will develop the force tool, characterize middle ear compliances in unimplanted human temporal bones, and perform initial evaluations of candidate coupling materials. Phase II will continue this work, conduct in vitro tests in implanted temporal bones, perfect the "soft contact" transducer, and evaluate new coupling materials such as hydrogels. Although directed primarily at implantable hearing aids, project fitidings are also applicable to any ossicular chain reconstruction prosthesis.